The rate of the Michael addition of piperidine to methyl propiolate in supercritical fluoroform and ethane at 37 degrees C and pressures between 48.3 and 213.8 bar depends on fluid density. In fluoroform, the rate constant is linearly related to pressure above 82.8 bar, with a smaller nonlinear change being observed at lower pressures and with a minimum at 82.8 bar. In ethane, the rate of reaction is linear with pressure except near the critical point where the rate is significantly enhanced. We attribute the observed rate constant changes to a dependence on solvent dielectric at pressures higher than that at the critical point, as would be consistent with the stabilization of a highly polar transition state. The observed aberrations near the critical point are attributed to solvent-solute clustering in fluoroform and to solute-solute clustering in ethane.